...and at long last: it wasn't a bug, it wasn't bad documentation, it wasn't BSD's fault in any way. The 540 in the 12+2 configuration has 12 drive slots on the front and 2 on the rear. Turns out that something, I have no idea what, means that while a SATA drive is recognised for boot (with UEFI) in the rear 2 plane, a SAS drive is not. It's a Weird Hardware Thing, and not something FreeBSD or the excellent handbook could, or should, ever have known about.
Now I'm off sorting out VNET jails to run Nextcloud.